Electronically programmable method for improving the control behavior of an anti-lock braking control system

ABSTRACT

The present invention relates to a method for improving an anti-lock control system, in particular for improving driving stability during braking on laterally different coefficients of friction. According to the method, a desired yaw rate is determined using at least one steering angle signal of a steering angle sensor and an actual yaw rate is determined using at least one yaw rate sensor, and the instability is evaluated using a parameter that serves for a qualitative and quantitative judgment of a deviation between the actual yaw rate and the desired yaw rate. Both yaw rate deviation and the time derivative of the yaw rate deviation are used to determine the parameter.

PRIOR ART

The present invention relates to a method for improving an anti-lockcontrol system, in particular for improving driving stability duringbraking on laterally different coefficients of friction.

The wheel rotational behavior is measured and taken into account fordetermining the wheel slip and for brake pressure modulation inprincipally known control methods. In the wheel-individual control(individual control, single wheel control) of vehicle wheels, the brakepressure of each wheel is controlled irrespective of the rotationalbehavior of the other wheels. Admittedly, short braking distances can beachieved with this procedure. E.g. on roadways with a laterally dividedfriction situation (μ-split), however, a yaw torque about a verticalvehicle axis may develop, necessitating active countersteering by thevehicle operator for compensation in order to prevent undesirable changein the driving direction. Not all vehicle operators consider themselvescapable of keeping an unexpectedly yawing vehicle to the track.

To reduce yaw torque caused by braking on laterally differentcoefficients of friction, a so-called yaw torque build-up retardation(GMA) has become known, which effects a retarded pressure build-up inthe wheel brake of a front wheel running on the road side with highercoefficient of friction (high-wheel) (Fahrsicherheitssysteme [vehiclesafety systems], 2nd edition 1998, Vieweg, page 53 et sqq.).

In vehicles with a less critical driving behavior the brake pressure atthe high-wheel is built up in steps as soon as the low-wheel due to alocking tendency undergoes a first pressure reduction. When the brakepressure of the high-wheel reaches its locking level, it is no longerinfluenced by the signals of the low-wheel but controlled individually.Only then will the possible brake force be utilized at this wheel.

In vehicles with a particularly critical driving behavior (short wheelbase, low moment of inertia, low rear-axle tire contact area), pressurebuild-up at the high-wheel will also follow pressure reduction andsubsequent pressure build-up on the low-wheel, with the pressurebuild-up pause times being by a defined factor longer than with thelow-wheel.

The prior art yaw torque build-up retardation (GMA) requires asophisticated adaptation to the vehicle concerned in order to reach acompromise between steering behavior and stopping distance. Said GMAsystem suffers from the shortcoming that the brake potential is notfully utilized, because the high-wheel is generally underbraked to agreat degree.

DE 42 08 141 C2 discloses an anti-lock control system for automotivevehicles processing signals from a yaw sensor system. Said known ABSsystem detects the yaw acceleration of the vehicle and adapts the slipratio between left and right wheels so that the yaw acceleration isreduced. Said system requires improvement because not all the drivingsituations are considered.

It is an object of the invention to overcome the shortcomings of thestate of the art. Another objective is to better utilize the brakepotential of the vehicle wheels, especially the high-wheel.

According to the invention, this object is achieved by the features ofpatent claim 1. The wheel rotational behavior is measured and used todetermine the wheel slip and for brake pressure modulation. In thisarrangement, driving stability is determined by using at least onesteering angle sensor for measuring a steering request and by using atleast one yaw rate sensor for measuring the vehicle yaw behavior, and isevaluated by way of a parameter Θ that is determined for qualitativelyand quantitatively judging a deviation between actual yaw rate ω_(act)and desired yaw rate ω_(LW) by employing the measured actual yaw rateω_(act), by employing a measured desired yaw rate ω_(LW), and byemploying the time derivative of the difference between actual yaw rateω_(act) and desired yaw rate ω_(LW).

ADVANTAGES OF THE INVENTION

With a view to influencing yaw torque, pressure modulation is executedespecially during ABS control intervention in dependence on a parameterΘ characterizing driving stability according to the invention. It ispossible to consider said parameter in the partial braking range—outsideof ABS control cycles.

The invention permits an adaptive design of the anti-lock pressuremodulation with influencing yaw torque in response to the degree ofinstability. The coefficient of friction available is better utilizedbecause pressure build-up times generally increased by a definedcoefficient are not envisaged on the high-wheel after pressurereduction. With short stopping distances, yaw torque is effectivelyinfluenced.

Further details of the invention can be seen in the subclaims inconnection with the description and the accompanying drawings. In thedrawings,

FIG. 1 is a flow chart of an embodiment of the invention.

FIG. 2 shows signal variations ω_(act), ω_(LW), Δω, Δω, and parameter Θas a function of time t.

FIG. 3 shows signal variations ω_(act), ω_(LW), Δω, Δω, parameter Θ aswell as pressure variations p_(VL), p_(VR) respectively as a function oftime t.

FIG. 4 shows a maximum allowable pressure difference on wheel brakes ofthe front axle in dependence on a yaw rate ω_(act).

DESCRIPTION OF AN EMBODIMENT

The method run will be explained schematically in the following by wayof a flow chart according to FIG. 1. The operation starts when accordingto a criterion (ABS₁₃ FA=1) mentioned at 1 an ABS control interventionis active on at least one wheel of the vehicle front axle. When thisdoes not apply, the procedure is discontinued. The parameterΘ—hereinbelow referred to as stability index—is produced according to 2.Included in parameter Θ are both the yaw rate deviation Δω and theacceleration deviation Δ{dot over (ω)} (time derivative of the yaw ratedeviation). A comparison 3 of the signs of parameter Θ and yaw ratedeviation Δω permits recognizing whether there is an understeeringtendency (4) when the signs are different (which the driver is stillable to master, as the case may be), which can be counteracted bypressure build-up modification 13, or whether there is a criticaloversteering tendency 5 of the vehicle when the signals are the same,which requires a pressure reduction modification 11.

In a following step 6 (FIG. 1), the wheel of the front axle isdetermined by way of parameter Θ (stability index) at which yaw torqueis influenced. For Θ<0 intervention is effected at the right front wheel7, while for Θ>0 intervention at the left front wheel 8 is initiated. Atreference numeral 9, the amount of the parameter Θ is taken into accountas a threshold in order to characterize the stability of the currentdriving condition. According to the present embodiment, an unstabledriving condition 10 prevails for parameters Θ>40, requiring pressurereduction modifications 11 at the identified highcoefficient-of-friction front wheel. The vehicle (i.e.|Θ|>Ysens_pdec_thr1=40) cannot be stabilized by pressure build-upmodification for Θ>40. A stabilizing pressure reduction modification 11is performed at a front wheel in this phase.

When the vehicle is unstable, the duration of a pressure reduction pulse(GMB_reduction pulse or PDEC_PULSE) and the duration of a pressurereduction pause (GMB_reduction pause or PDEC_PAUSE) between neighboringpressure reduction pulses is calculated as follows:

${{PDEC}\_{PULSE}} = \frac{\Theta }{{{Ysens}\_{pdec}}{\_{pulse}}{\_{quotient}}(8)}$${{PDEC}\_{PAUSE}} = \frac{{{{Ysens}\_{pdec}}{\_{thr}2}} - {\Theta }}{{{Ysens}\_{pdec}}{\_{pause}}{\_{quotient}}(8)}$

In the above equations the parameter Θ is variable, while the otherquantities are fixed. Introducing the determined duration into thecontrol speeds up the pressure reduction at the highcoefficient-of-friction wheel and thereby reduces undesirable yawtendencies. A GMB reduction pause (Min(GMB_reduction pause)) lasts atleast five loops long. This minimum reduction time is necessary in orderto obtain the reaction of the vehicle to the given pulse. When thevehicle has returned to the stable range, pressure build-up modification13 may be performed in order to improve brake performance.

For plausibility reasons, a pressure reduction modification is onlyperformed when the result of a comparison 12 is that the wheel brakepressure at the vehicle wheel intended for yaw torque influencing, whichpressure is determined from wheel-individual slip values on the basis ofa pressure model, is higher than the wheel brake pressure (modelpressure) determined on the opposite vehicle wheel. When thisplausibility condition is not satisfied, switch-over to a pressurebuild-up modification 13 is made which is generally provided whenvehicle 14 is stable. Within the pressure build-up modification 13, abuild-up pause (ABS_build-up pause) is determined on the basis of thepressure model in consideration of the wheel slip condition, and abuild-up pause (GMB_build-up pause) is determined on the basis of yawtorque influencing in consideration of the parameter Θ, bothdeterminations being made irrespective of each other. The determinedpause times are compared, and the longer pause time is input into thecontrol. The input minimum build-up pause (GMB_build-up pause_min)principally amounts to about 2–3 loops, that means, betweenapproximately 14–30 ms depending on the internal clock time. A minimumpause of 7 loops is adjusted only at the commencement of the controlwith high yaw rates (>10°/s). The purpose of this special minimum pauseis to ensure higher stability in curves from the very beginning. Thealgorithm ends after having established the necessary pressuremodification, and, if necessary, a new calculation loop will start.

FIG. 2 illustrates by way of signal variations vehicle instabilitycaused by a braking operation. In the top part a desired yaw rate ω_(LW)measured at the steering wheel is depicted in relation to the measuredactual yaw rate ω_(act). At time t₁ the vehicle becomes unstable anddisplays a yawing tendency (starts skidding). As can be seen, thishappens independently of the driver's wish (ω_(LW)) because no steeringangle variation is introduced. The yaw rate ω_(act) rises until time t₂and will decline until t₃ caused by a countersteering maneuver initiatedat time t₄. The vehicle changes the yaw direction starting with t₄. Themiddle part of FIG. 2 illustrates the deviation between actual yaw rateω_(act) and desired yaw rate ω_(LW) (in other words: the yaw ratedeviation Δω) as well as the time derivative of this deviation(acceleration deviation Δ{dot over (ω)}). The calculation of parametersis essentially based on geometrical addition. The bottom part of FIG. 2shows exemplarily a parameter (stability index) Θ determined from testvalues and weighting the yaw rate deviation Δω and the accelerationdeviation Δω by way of the coefficients P and D, which are adjustable ina vehicle-related fashion. The result is a PD controller, and theparameter Θ can be used for stabilization. The vehicle is consideredunstable only in the period between t₅ and t₆ because the parameter Θexceeds the value 40.

Principally a distinction is made between different scenarios withpressure build-up phases and pressure reduction phases depending on theABS control condition of the vehicle within yaw torque influencing(GMB).

With ABS control on one side, unsymmetrical friction value differencesare assumed, due to which different brake forces can be applied. Thedifferent forces induce a yaw torque about the vertical axis. There is ahigh-sensitivity reaction to instability in order to render it possibleto the driver to react with gentle steering maneuvers. Initially, thelow-wheel will undergo ABS control. The vehicle passes through a firststabilizing phase. In this phase, development of a first critical yawrate amplitude is prevented by adjusting a defined pressure differenceon the front axle depending on the ‘actual’ yaw rate level. FIG. 4 showsthe maximum allowable pressure difference in dependence on the actualyaw rate (Ysens_fpd_press_diff) under the condition that {dot over (ω)}exceeds an absolute value of 6°/s². With rising yaw rate ω until roughly10°/s there will be a linear reduction of the maximum allowable pressuredifference on the front axle. Commencing in about 10°/s the allowablepressure difference remains constant until 15 bar approximately, yet thepressure difference is allowed to exceed or fall under this value independence on the vehicle, the desired adaptation, and tolerances.

The above-described yaw rate dependency considers the stronger tendencyto instability of vehicles, which exhibit a yaw rate ω (e.g. due to lanechanging or cornering maneuvers) already before the control begins.Vibration of the vehicle about the vertical axis is dampened by pressuremodulation at the front wheel of the high coefficient-of-friction side,and the yaw rate deviation Δω is increased adequately. This gives thedriver an opportunity of countersteering. On the highcoefficient-of-friction wheel the brake force can be increased untilthis wheel also reaches its locking pressure level and enters into ABScontrol.

Referring to an initially one-sided ABS control intervention due to alow coefficient of friction on the left front wheel, FIG. 3 illustratesthe yaw torque influencing method of the invention by way of thevariations of desired yaw rate ω_(LW), actual yaw rate ω_(act) ,parameter Θ as well as the associated pressure variations on thehigh-wheel p_(VR) and the low-wheel p_(VI). Because the sign ofparameter Θ is negative in point a, GMB becomes active with a reductionon the right front wheel (high-wheel). As becomes apparent from thepressure variations, wheel-slip-induced pressure reduction cycles b areadjusted on the low-wheel. Pressure reduction cycles c are performed onthe high-wheel for reducing the pressure difference that develops. Thispressure reduction takes into consideration the actual yaw rate ω_(act),as can be seen in FIG. 4. The following pressure build-up and pressurereduction cycles are based on an interaction of parameter and slipthresholds. Both wheels of the front axle are in ABS control at time t₀.

A declining parameter Θ between the points of time t₁ and t₂ renders aquasi sneaking instability tendency of the vehicle apparent. Accordingto the invention, the reaction to this condition within GMB is avariation (extension) of the pressure build-up pauses d, e, and f on thehigh-wheel. When the parameter Θ at time t₂ reaches its minimum, aparticularly long pressure build-up pause f obviously prevails.Following another pressure build-up g is then a pressure reduction hthat is basically due to wheel slip. Moreover, the pressure variationsp_(VR) and p_(VL) permit detecting the pressure difference between thetwo wheel brakes of the front axle that rises in the course of the ABScontrol cycles, allowing an improved brake effect.

The present invention permits detecting stable and unstable phases withgreater reliability because the yaw rate sensor and the steering anglesensor provide signals that allow a precise adjustment between nominaland actual values.

Shorter stopping distances on μ-split roadways can be realized becausethe stable phases within ABS can be better utilized. Another advantageis the benefit gained in stability when braking in cornering maneuversbecause pressure modulation on the curve-inward wheels helps the vehicleto maintain its track. Oversteering tendencies are thereby avoided.

Very unstable situations within ABS control are obviated and skidding ofthe vehicle is prevented so that ESP interventions within ABS controlare mostly avoided.

The parameter (stability index) Θ is a central issue of the invention.It is possible within ABS control due to the stability index formed fromthe yaw rate deviation Δω and the acceleration deviation Δ{dot over (ω)}to early detect tendencies of the vehicle to instability and to reactaccordingly in connection with the described GMB method. The control isa closed-loop control in contrast to a superimposed ESP control thatinterrupts the ABS control for a brief interval.

Evaluations have shown that the parameter Θ plausibly represents vehicleconditions (oversteering, understeering). This result renders itpossible to implement the invention in the described form or in modifiedform for further fields and conditions of application of electronicvehicle control systems (such as ABS, ESP, ESBS, EMB), in particular forthe partial brake range. It is self-explanatory that in an adaptationfor the partial braking range step 1 in FIG. 1 (ABS₁₃ FA=1) is notpolled.

Finally, it should still be pointed out that all mentioned numericalvalues are meant exemplarily, and that in each case there is a top and abottom tolerance range for adapting to the respective type of vehiclewithout departing from the essence of the invention.

List of Reference Numerals: ω_(act) (actual) yaw rate ω_(LW) desired yawrate Δω yaw rate deviation Δ{dot over (ω)} (yaw rate) accelerationdeviation Θ parameter (stability index) p_(VR), p_(VL) wheel brakingpressure front right, front left t time P, D coefficients a point bpressure reduction cycles c, h pressure reduction cycles d, e, f, gpressure build-up pauses  1 step  2 step  3 comparison  4 understeeringtendency  5 oversteering tendency  6 step  7 right front wheel  8 leftfront wheel  9 step 10 unstable driving condition 11 pressure reductionmodification 12 comparison 13 pressure build-up modification 14 stablevehicle

1. A method for improving an anti-look control system, in which thewheel rotational behavior is measured and taken into account fordetermining the wheel slip and for brake pressure modulation, whereinthe method includes the steps of determining driving stability by usingat least one steering angle signal of a steering angle sensor fordetermining a desired yaw rate (ω_(LW)) and by using at least one yawrate sensor for measuring an actual yaw rate (ω_(act)), and evaluatingthe driving stability by way of a parameter (Θ) that is determined forqualitatively and quantitatively judging a deviation between actual yawrate (ω_(act)) and desired yaw rate (ω_(LW)) by employing the measuredactual yaw rate (wood and by employing a measured desired yaw rate(ω_(LW)) and by employing the time derivative of the difference (Δ{dotover (ω)}) between actual yaw rate (ω_(act)) and desired yaw rate(ω_(LW)).
 2. The method as claimed in claim 1, wherein the parameter (Θ)is determined according to the formula Θ=P*Δ{overscore(ω)}+D*Δ{overscore ({dot over (ω)} with Δω=ω_(act)−ω_(WL) (yaw ratedeviation) and${\Delta\omega} = {{\Delta\omega}\frac{\delta}{\delta\; t}}$(acceleration deviation) and with P, D as vehicle-related coefficients.3. The method as claimed in claim 1, wherein a brake pressuremodification is effected through pressure build-up pulses and pressurereduction pulses, and the driving behavior is distinguished by way ofthe amount of parameter (Θ) in a stable or an unstable range indicatingthe extent the vehicle is oversteering or understeering, and when thevehicle is stable, a modification of the pause time (GMB_build-up pause)between respectively adjacent pressure build-up pulses is performed forthe pressure build-up modification (13) on that wheel of the front axlethat has the higher coefficient of friction with the mad, and whereinwhen the vehicle is unstable a pressure reduction modification (11) isperformed by modifying the pressure reduction pulse length(GMB_reduction pulse), or a modification of a pause time (GMB_reductionpause) is performed between respectively adjacent pressure reductionpulses.
 4. The method as claimed in claim 3, wherein, when the vehicleis stable, a pressure build-up modification (13) with a pause time(GMB_build-up pause) is determined on the wheel with the highercoefficient of friction on the basis of the parameter (Θ) forcharacterizing driving stability (Θ) and a pause time (ABS_build-uppause) on the basis of wheel slip, wherein the determined pause timesare compared, and wherein the longer one of the determined pause times(Max(ABS_build-up pause, GMB_build-up pause) is used for pressuremodulation on the wheel with the higher coefficient of friction.
 5. Themethod as claimed in claim 3, wherein, when the vehicle is unstable, apressure reduction modification (11) with a pause time between adjacentpressure reduction pulses (GMB_reduction pause) and/or a pressurereduction pulse length (GMB_reduction pulse) is determined on the wheelwith the higher coefficient of friction on the basis of the parameter(Θ) and used for pressure modulation.
 6. The method as claimed in claim5, wherein the pause time (GMB_reduction pause) is determined accordingto the formula (GMB_reduction pause=GMB_reduction_thr2−|Θ|/GMB_reductionpause_quotient.
 7. The method as claimed in claim 5, wherein thepressure reduction pulse length (GMB_reduction pulse) is determinedaccording to the formula GMB_reduction pulse=|Θ|/GMB_reductionpulse_quotient.
 8. The method as claimed in any one of claims 3, 5, 6,7, wherein, when the vehicle is unstable, modification of pressurereduction is not performed on the wheel with the higher coefficient offriction, and wherein modification of the pressure build-up is performedwhen the wheel brake pressure (Pmod_Wh_YTC) determined according to apressure model is lower on the wheel with the higher coefficient offriction than the wheel brake pressure (Pmod_Wh_No_YTC) determinedaccording to the pressure model on the wheel with the lower coefficientof friction.
 9. The method as claimed in claim 3, wherein, forparameters (Θ) ≦40 on the wheel with the higher coefficient of friction,a pressure build-up modulation (13) for a stable driving condition iseffected, and wherein a pressure reduction modulation (11) for anunstable driving condition is performed for parameters (Θ) roughly ≧40.10. The method as claimed in claim 1 wherein the direction ofinstability (oversteering tendency/understeering tendency) is determinedby comparing the signs of parameter (Θ) and yaw rate deviation (Δω), andwherein identical signs signal an oversteering tendency, and differentsigns signal an understeering tendency.
 11. The method as claimed inclaim 1, wherein the vehicle wheel intended for yaw torque influencingis determined by the sign of the parameter (Θ), and wherein a positivesign brings about yaw torque influencing on the left front wheel (VL),and that a negative sign brings about yaw torque influencing on theright front wheel (VR).